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Old Streetfighter Motorcycle Forum Post 08-17-2019, 11:50 PM   #1
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Default Rectangular steel tubing bend procedure part one

Rectangular steel tubing bend procedure.


The thread Mongoose started struck a cord with me as I have often done this sort of thing in engineering; architectural; motorcycles; and artistic, where with the help of my cousin, a quite accomplished structural artist and fabricator, we first figured out this procedure.


Getting started
See illustration below.

The first thing you need to do is draft up an accurate template of the stock frame and wheel setup, particularly the width of the wheel assembly including the brake assembly and how the braking loads are fed to the frame or swingarm, the width of the frame mount points, the relative distance of the frame pivot to the axle with the chain adjusters in mid range and the anticipated travel of the adjusters. With all that information it is relatively simple to knock out the basic layout of the main arms, and most importantly the necessary angle, desired radius, and location of the bends. You knew I would get there eventually, right? Once there the fun starts.
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Initial Layout
See illustration below.

When drafting the bend cutouts with CAD or a drafting board (does anyone still have one of those?) it is easiest if you use the center of the bend as a vertical (y) origin and the center of the bend radius as the horizontal (x) origin, and the profile a V shape with both sides half of the desired final angle. For ease of illustration Im using 2 x 3 inch tubing with a wall thickness of 0.083 inch, a radius of ten inches, and a bend angle of twenty degrees. Draft two lines (Red) of a length of ten inches from the origin at two hundred sixty and two hundred eighty degrees respectively. A curve of ten inches minus the stock wall thickness including the total width of the chamfer drafted at the origin to the two lines is the radius of the cuts you need to make (Green). Now draft a second radius of ten inches to twenty degrees centered on the origin (Blue). This is the outside of the bend. Draft two straight lines ten inches long tangent to the arc you just drafted to establish the outer constraint (Magenta), then two lines two inches long to the ends of the outer constraint lines at a right angle (Cyan). Next, draft two lines parallel to the outer constraint until they meet at y = 0 (Orange), generating the inner constraint. Just one more line from the junction of the two Orange lines to the center of the Green arc (Black). When flattened out this is the path for the center cuts.
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Finishing the cutting pattern
See illustration below.

In order to make a useful pattern it is necessary to flatten out the draft you just did. This is where the elementary math mentioned before comes into play. Using the formula Length = Radius x Angle x ( / 180), determine the actual length of one of the Green arcs generated previously. Plugging the values into the formula thus: 9.834 x 10 x .01745329 = 1.716357 inches. Double that and you have the length of the straight lines replacing the arcs previously drafted. Rotate the Magenta lines on the left side down ten degrees, and the Brown lines on the right down ten degrees as well. Join the sides with the calculated length lines and the pattern is ready to be printed and cut out. Using heavy photo paper print two on the same sheet, one for the top and the other for the bottom. When cutting out leave a wide section of the center of the V connected. Once they are both cut out invert one (printed side down), lay them on top of each other, carefully align the cutout patterns, then trim the ends so both are exactly the same length. This facilitates aligning the patterns when applying them to the stock.
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Applying patterns to the stock
See illustration below.

First, clean and degrease the stock in the area where the patterns are to be applied. Use acetone for a final wipe.
It is of paramount importance to apply the patterns accurately to insure the alignment is as exact as possible, and to that end I use two rectangular pieces of 1 x 6 wood cut exactly to right angles clamped to the three inch sides of the stock and carefully aligned to each other across the stock, but also an exact as possible right angle to the horizontal surface, a two three four block works well for this, then c-clamp them to the stock. This serves as an accurate location when placing the patterns. To apply the patterns I use a 3M product called Spra-Ment: a mild spray adhesive commonly available at Michaels, Hobby Lobby, and pretty much any craft store worth its salt. Spray an even layer on the non printed side of the pattern, allow to set for the time specified in the directions, then place a 6 inch steel scale against the edges of the boards and butt the edge of the pattern against the scale, then roll it down until it is firmly adhered to the stock. No need to adhere it to the chamfer. Repeat on the other side. When the adhesive is cured remove the wood and cut out the piece of paper inside the V on both sides using a scalpel and straightedge but do not peel them off yet. Place a straightedge on the long side of the stock and line it up with the edges of the top and bottom V cuts, then scribe across the stock. Clean up any residual adhesive. You are now ready to mark the stock. Using a good sharp scribe score the stock all the way around the cutout pattern. It is not possible to accurately scribe across the chamfer, so dont bother. Once that is done spray paint the top and bottom avoiding the scribed marks on the wide side.
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Elaboration on the chain adjusters


If you noticed the eccentric chain adjusters in the first illustration look a bit different, they are. The one problem with eccentric adjusters is while adjusting the chain you are also changing the ride height. In an effort to overcome this I came up with double eccentric adjusters which allow adjustments in both chain tension and ride height independently. Actually, it was originally designed for a custom GSXR frame with the adjusters mounted to the pivot which allowed fine adjustments to the pivot / countershaft angle and distance to optimize anti squat and mechanical traction. Worked like a charm. That, and it was fun to draft it up.
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That's it for now.

Rob
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Old Streetfighter Motorcycle Forum Post 08-19-2019, 12:11 PM   #2
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Default Re: Rectangular steel tubing bend procedure part one

The part that seems the most difficult to me is cutting the cutouts near the outside of the bend. How do you accurately do that?

A steering damper dampens steering oscillations. If it starts to leak, it may become a steering dampener. There's no such thing as a dampner.

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Old Streetfighter Motorcycle Forum Post 08-19-2019, 01:16 PM   #3
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Default Re: Rectangular steel tubing bend procedure part one

Good stuff,man! Rep'd for sharing this
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Old Streetfighter Motorcycle Forum Post 08-19-2019, 03:43 PM   #4
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Default Re: Rectangular steel tubing bend procedure part one

That is the most difficult part of the entire project, and the aspect that took the longest to get right. With the advent of ever better CNC LASER cutters it is possible to make a consistent cut all the way to the apex of the two lines with a total cut width less than 0.001 inch. Of course when I figured out this procedure LASERs of any sort were the stuff of science fiction, at least for the average Joe, and it took lots of experiments and failed parts to get it right.

Essentially, when the patterns are applied to the stock center punch the ends of the patterns circled in red in the illustration, then when the patterns are removed drill at the center punched location with the smallest drill you can obtain, my preference is a #60. It's easy to break especially when drilling steel so spin it fast and feed it slow, frequently backing it out to clear chips. This gives a good target to end the cut. Continue to drill progressively smaller holes (Green in the illustration) until you can't guarantee that they will stay inside the pattern. To cut out that area I use a Dremel with a #545 diamond wheel, the thinnest they have at 0.023 inch, and tough as hell. Using the lower boundary as a guide cut until the wheel gets to the drilling. Once that is done on both sides use the cutoff wheel to continue the cuts all the way across the bottom of the pattern. The same wheel can be used to cut just under the radii, then brought to the scribed line of the radii first with a fine carbide burr and when that gets too big: needle files. The initial straight cut will leave a 0.023 inch gap (if you have a steady hand) about 3/4 inch long (horizontal Red line on the illustration) that you can't do anything about, but this will have minimal effect on the bend, and will be filled when welded.

When I get a CNC LASER cutter it will take almost a minute to cut. Hey, I can dream, can't I?

Finishing writing the actual bend procedure. Some small details that can snatch success from the toothy maw of failure. Will post it probably tomorrow.

Rob
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Old Streetfighter Motorcycle Forum Post 08-21-2019, 09:25 AM   #5
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Default Re: Rectangular steel tubing bend procedure part one

Finishing the cutting, finer points, and bending


Finishing the cutting
Having already cut out the hard part, the only cuts left are for the V, which after the picky work youve already done will be a cakewalk. Cut it out staying within the lines. Whee!

Bending

The first attempts at bending were frustrating, but informative. Given the complication of not having a flat piece of metal which would be a cinch to bend, it includes the entire chamfer which comprises a brace to the bulk of the metal being bent. Usually, it would cone outward, warping the flat side inward. Bummer. Attempts at constraining the chamfer with thick plates c-clamped to it only caused both to buckle. After working on it for an intermittent month or so we agreed to give it another go or die trying, meeting at a local watering hole. A couple of good strong ales each in a decent simulation of an English pub later my cousin and I came up with a process that in our partially inebriated state made sense. All we needed to do is directly bend the outer wall with a piece of three inch thick oak plank thinned to a loose slip fit inside the tubing and with a radius matching the desired inner bend backed up with a steel shoe with a cut down half inch socket welded to it.
Additionally, we chamfered the lower part of the cuts for welding which we hoped would considerably reduce the buckling, and all the joining cuts as well with the exception of the radius cuts to keep a full width surface the outer metal could bear against. We slid the tool in, engaged the socket with a necked down short heavy duty extension, supported the stock on two inch oak dowels, put it in his press, and pumped it down. To our surprise it actually worked! The outside was flat, and while there was some slight outward coning of the chamfer it was easily worked back into place with a body hammer and buck while still in the press. Of course the bend could not be completed due to the extension in the way, but it appeared that the initial bend made the rest more amenable to cooperate with a ratchet tiedown pulling on each end while on its side on the surface table. Once the desired bend angle was reached I chamfered the radius cuts, tacked it together with a TiG, and handed it to his certified welder to complete. After Thor knows how many tries it finally worked! Not nearly a steep enough learning curve. At least we got very good at the picky little cuts and concentration required which has impressed itself on my motor cortex to great effect in the decades since, especially when working on surface mount electronics.

Next, gusseting. The inside of the bend is a sharp angle which is a no-no in terms of load bearing, and lets be honest, looks. Solving the load problem while making it look truly workman hewn is worth doing.

Rob

If a moderator could delete the duplicate post #5 Id appreciate it.
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Old Streetfighter Motorcycle Forum Post 08-27-2019, 08:57 PM   #6
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Default Re: Rectangular steel tubing bend procedure part one

https://youtu.be/be4LrGn0lPg
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